In general, phase change inks are in the solid phase at ambient temperature, but exist in the liquid phase at an elevated operating temperature of an ink jet printing device. At the jet operating temperature, droplets of liquid ink are ejected from the printing device and, when the ink droplets contact the surface of the printing material, they quickly solidify to form a predetermined pattern of solidified ink drops.
Because phase change inks remain in a solid phase at room temperature, they facilitate shipping and long-term storage. Among other numerous advantages, these inks also largely eliminate nozzle-clogging problems caused by ink evaporation, and thereby improve the reliability of ink jet printing.
Phase change inks have been employed in both direct and transfer printing processes. A phase change ink composition is typically cast into solid ink sticks and placed into an ink jet printing device. Then, the temperature is raised to a first elevated operating temperature where a liquid phase with selective fluid properties is formed. The ink is then typically held as a liquid at this relatively high operating temperature in a reservoir and printhead of an ink jet printer.
The liquid phase ink composition can then be applied in a predetermined pattern onto a substrate. In transfer type printers, for example, the ink composition is deposited onto an intermediate transfer surface such as disclosed in U.S. patent application Ser. No. 07/981,646, filed Nov. 25, 1992, assigned to the assignee of the present invention, and herein incorporated by reference. The intermediate transfer surface is held at an intermediate temperature, which is below the melting point of the ink formation, but is above ambient temperature. At this intermediate temperature, the ink composition is maintained as a solid, but is malleable and has specified mechanical properties that enable it to be used in subsequent steps of the process.
The ink is then transferred in an "imagewise" or "pagewise" fashion to the final substrate by use of a pressure nip where the specified pressure is above the compressive yield strength of the solid, malleable ink at the intermediate temperature mentioned above. Preferably, the final receiving substrate or surface is heated to a temperature greater than the intermediate temperature before being fed into a nip where the substrate is brought into contact with the malleable ink droplets to form a desired image or pattern. During this transfer step, the ink droplets are flattened, spread, and in the case of paper substrates, fused into the substrate. The final step in the process is removal of the final substrate from the pressure nip and the separation of the substrate and ink layer from the intermediate transfer surface. During this step, the ink must retain enough cohesive strength to resist cohesive failure due to the tensile forces it experiences as it is peeled from the transfer surface. Thin films of uniform thickness of the phase change ink composition on the final receiving substrate when cooled to the ambient temperature must be ductile and retain sufficient flexibility so that the image will not fracture upon bending, while possessing a high degree of lightness, chroma, transparency, and thermal stability.
A phase change ink composition must have process compatible fluidic and mechanical properties in order to produce a printed substrate of high quality. Desirable properties of solid state phase change ink compositions are specified and measured by using several analytical techniques. One such technique is dynamic mechanical analyses (DMA), which measures the viscoelastic properties of a material by identifying the material's elastic and viscous components. The measurements are made by subjecting the ink composition to an alternating (oscillatory or dynamic) strain, and simultaneously measuring the alternating stresses and phase angles at different frequencies and temperatures. The dynamic stress (s*) of an ink composition can be separated into two components. There is the "elastic stress" component which is the magnitude of the portion of the applied force in phase with the applied strain, and the "viscous stress" component, which is the magnitude of the portion of the applied force out of phase with the applied strain. The dynamic modulus (E*) can be determined from the ratio of dynamic stress over strain. Correspondingly, it can be broken down into the in-phase component of the modulus, E', and the out-of-phase component of the modulus, E". E' defines the energy stored in a specimen under an applied strain. E' is determined by the equation E'=s./E. (Cos .delta.). E" defines the energy loss due to viscous dissipation under an applied strain. E" is determined by the equation E"=s./E. (Sin .delta.).
The phase angle (.delta.) is the lag in the measured stresses to an applied strain due to the viscoelastic nature of the material. The loss tangent (tan .delta.) is the ratio of loss modulus over storage modulus. Tan .delta. is often referred to as the dissipation (damping) factor, i.e., the ratio of energy dissipated per cycle to the maximum potential energy stored during a cycle. Finally, the glass transition temperature (T.sub.g) is a temperature at which there is a large drop in modulus, about 0.5 to about 3 orders of magnitude, accompanied by a definite peak of the tan .delta.. Below the T.sub.g, the material behaves like a brittle solid. At the T.sub.g, the material acts as a leathery solid and is capable of dissipating energy. Above the T.sub.g, the material is similar to a rubbery solid. Dynamic properties are usually plotted in terms of E', E" and tan .delta. as a function of temperature at a constant frequency or as a function of frequency at a constant temperature. Through the entitled, "Viscoelastic Properties of Polymers," Chapter 11, pages 264-320, 3rd Edition by John D. Ferry; it is understood that the effect of changing temperature will correspond to a shift along the frequency axis for all relaxation processes, without appreciable change in magnitude of all of these relaxation processes.
Another of the mechanical analytical techniques mentioned above is compressive yield testing on bulk samples of the phase change ink compositions. Yield stress is the point on the stress-strain curve at which the material continues to deform without an increase in stress. This is important in the printing process mentioned above since it determines the pressure needed to spread the solid, malleable ink droplets into a continuous thin film during the transfer process.
There are various types of deformation the ink undergoes in compression as a function of temperature or rate. An ink can be classified as being brittle if it fails by the shearing and fracturing of the molecular bonds. This is typified by low elongation (which is directly proportional to strain) and moderate to high stress. Since the integration of the area under the stress-strain curve is a measure of the toughness of the material, a brittle material is strong, but not tough. The brittle behavior is detrimental to the durability of the ink on substrates because it is both low in elongation (i.e., not very ductile or flexible) and toughness (i.e., the ability to dissipate energy).
An ink is considered to be ductile if it fails by sliding the molecules over each other and flowing. This is typified by high elongation and toughness. Ductile behavior is desirable for a printing process involving transfer and fusing or fixing because it allows the ink to spread by flowing under an applied pressure without fracturing.
Shear banding is the transition between the ductile and weak behavior, where the ink loses its cohesive strength. Shear bands are typified by 45.degree. angle criss-crossed bands that indicate the ink is weakening. Weak behavior is characterized by the crumbling behavior of the ink. This is due to the loss in cohesive strength of the material. It is theorized that this occurs once short molecules have flowed past one another at high elongation. The weak behavior is to be avoided during the image transfer and fusing steps because it leads to poor durability of the ink on substrates, poor rectilinear light transmission of the ink, and poor transfer efficiency during printing.
Phase change inks typically include a phase change ink carder composition that is combined with a phase change ink compatible subtractive primary colorant. The subtractive primary colorants may, for example, comprise cyan, magenta, yellow, and black component dyes that are typically selected from either class of Color Index (C.I.) solvent dyes, dispersed dyes, modified acid and direct dyes, and a limited number of basic dyes.
The ingredients in phase change ink compositions are selected to achieve desirable print quality and process stability. Colorants or dyes are often selected for light fastness, as well as hue, brightness, thermal stability, rectilinear light transmissivity, co-compatibility, and other criteria. For example, fluorescent dyes tend to be bright but tend to complicate ink color matching, and are not tremendously light fast. Some xanthene dyes, such as certain rhodamine dyes, tend to be fluorescent, but somewhat light fast; however certain of these dyes undergo a facile rearrangement between two different structures, only one of which is colored. Anthraquinone dyes, on the other hand, are extremely stable to both heat and light but tend to migrate or separate from the ink compositions to form a layer of powdered dye on print surfaces. This phenomenon, called "blooming," may take considerable time to become evident and can ruin the color and aesthetic appearance of prints.
Carrier compositions are often selected for specific fluidic and mechanical properties such as those described in U.S. patent application Ser. No. 07/981,677, filed Nov. 25, 1992, assigned to assignee of the present application, and herein incorporated by reference. The composite phase change ink compositions must be compatible with a selected ink jet printing process, such as the above-described transfer printing process, while retaining the desired colorant characteristics. Exemplary phase change ink compositions, including colorants and carrier compositions, are disclosed in U.S. Pat. Nos. 4,889,560 and 5,084,099, which are herein incorporated by reference.
The printing industry is ceaselessly attempting to increase printing speed, increase the interval between ink reloading, decrease the cost per printed substrates, and reduce the size of ink jet printers (including printheads and reservoirs). One strategy for facilitating these improvements includes decreasing the amount or film thickness of phase change ink ejected from the printer and transferred to the substrate. Thinner ink films would also decrease the thickness of printed ink above the substrate surface, reducing the likelihood of abrasion or blocking.
However, ink reduction strategies impose even greater constraints on phase change ink compositions. If the printed ink quantity is halved, for example, then the colorant concentration in the ink composition must be substantially increased to maintain desirable printed color saturation. The limited solubility of many colorants in carrier compositions generally does not permit sufficient increase of the specific colorant concentration to satisfy printed color quality, including intensity.
It would therefore be advantageous to devise for selected printing applications an ink composition and method that facilitate a reduction in printed ink quantity and maintain color intensity.